The characteristics of light emanating from an object or a sample may be advantageously detected in order to determine characteristics of the emission source. For many years, spectrographic techniques have been used to perform analysis of materials ranging from human blood and other biological materials to slag from a crucible. For example, it has been known that wavelengths of light absorbed by a material, as well as the wavelengths of light emitted by a material during an excited state, such as combustion, both indicate the composition of the material. Today, analytic instruments in industrial, scientific and medical applications make widespread use of such emission spectra and absorption spectra. Other techniques for exciting molecules to emit light include formation of a plasma. Causing light to fall upon a material to be analyzed may also be used to stimulate emission of light for spectrographic analysis. Such techniques include Raman spectroscopy, where, for example, the output of a mercury vapor arc may be filtered and used to excite a transparent sample. As the light passes to the sample, it is scattered and undergoes a change in wavelength and a random alteration in phase due to changes in rotational or vibrational energy of the scattering molecules. Raman scattering is a principal analytic tool in industry and science today.
Another class of analytic instruments uses fluorescence measurements to identify materials. In such systems, an excitation source, such as a laser, is used to excite atoms or molecules, raising electrons into higher energy states. When the electrons revert back to the unexcited state, they fluoresce or emit photons of light characteristic of the excited atom or molecule. The wavelength of the emitted light thus contains information respecting the identity of the excited atom or molecule. In addition, the delay between the exciting light and the emitted light, as well as the amplitude of the emitted light, both give information respecting the excited atom or molecule.
While one may visualize an excitation pulse of light being pumped into a sample and the emission spectra measured and analyzed over time, in practice, such measurements are achieved by causing a light source from an excitation source modulated with a radio frequency periodic signal to excite a sample. In particular, a pulsed dye laser, or a continuous wave laser whose output is modulated by a Pockels cell, may be used to excite a sample to fluoresce. In such systems, the modulated laser excitation source is caused to fall on the sample. The modulated laser excitation source comprises a pencil of light modulated in intensity at the laser intensity modulation frequency. After a very short period of time, the system reaches the steady-state. During the steady-state, a steady-state fluorescence emission occurs. This steady-state fluorescence emission may be measured to determine the phase and modulation of the emission as compared to the excitation.